Stabilized stannous fluoride toothpaste

ABSTRACT

Stannous fluoride containing toothpaste compositions including an alkyl vinyl ether and maleic anhydride or acid copolymer as a stabilizing agent for stannous fluoride and methods for preparing such stabilized compositions.

FIELD OF THE INVENTION

This invention relates to a stannous fluoride toothpaste with improvedstability. In particular, this invention is directed to an aqueoustoothpaste composition comprising stannous fluoride stabilized with acopolymer of maleic anhydride or acid and a lower alkyl vinyl ether.

BACKGROUND DF THE INVENTION

Stannous fluoride has been used in oral care products since the early1950's, and stannous fluoride has been reported to be an effective agentfor treating various oral conditions and diseases including plaque,gingivitis, sensitivity, enamel decalcification, and periodontitis,among others. Product stability is an outstanding problem becausestannous fluoride is unstable in water and forms stannous oxy-fluorideand stannic compounds, both of which reduce or inhibit enamelfluoridation, that is, formation of stannous fluorophosphate andfluoroapatite. This instability has required the withdrawal andreformulation of the original stannous fluoride toothpaste as a sodiumfluoride composition. It has required that stannous fluoride products beformulated as non-aqueous compositions such as powders and non-aqueousgels in moisture-free conditions. Despite highly specializedprecautionary measures during formulation of oral care products,significant variations are found in stannous ion concentrations ofcommercial dentifrice gel products among different manufacturers as wellas among batches of the same brand of dentifrice because of stannous ioninstability.

DESCRIPTION OF THE PRIOR ART

The original aqueous stannous fluoride toothpastes have beenreformulated as stannous ion-free compositions because of theinstability of the stannous ion in water. Numerous stannous fluorideformulations with "stabilizing agents" have been proposed.

U.S. Pat. No. 3,445,567 claims that aqueous stannous fluoridecompositions can be stabilized with sorbitol or a mixture of sorbitoland glycerine. However, this composition lacks the shelf-life stabilityrequired for commercial use.

U.S. Pat. Nos. 3,711,604, 3,919,409, 3,935,306 and 3,980,767 disclosetoothpaste and dentifrice formulations including soluble fluoridecompounds, including stannous fluoride. The toothpaste formulations usea variety of standard ingredients and gelling agents includingcarboxyvinyl polymers sold under the tradename Carbopol and insolubleabrasives such as silica and silicates. The stannous fluoride containingembodiments disclosed in these patents lack the stability found in thecompositions of this invention.

U.S. Pat. No. 4,418,057 describes another approach. Stannous fluoride isformulated as a non-aqueous gel mixture including anhydrous glycerin andhydroxyethyl cellulose gelling agent. Total exclusion of moisture fromthe moisture is required to protect the stannous ion.

U.S. Pat. No. 4,259,316, while noting the instability of stannousfluoride, proposes a compositions containing phytic acid for inhibitingdental caries. Both dry dentrifices and aqueous formulations aredescribed. If protected from moisture, the dentrifices would appear toretain the stannous ion in efficacious form. The stannous ion wouldrapidly convert to the ineffective oxy-stannous or stannate forms uponexposure to moisture in solution or in the oral cavity. Apparently,excess amounts of stannous fluoride are added in an effort to offset theloss of stannous ion. However, this requires the exposure of the user toexcessive, unsafe levels of stannous ion.

EPO application 88308337.0 (Publication No. 0 311 260 A3) publishedApril 12, 1989 describes compositions containing stannous fluoride, andas a stannous reservoir to replace stannous ion lost throughdegradation, stannous gluconate. Listed as suitable abrasive polishingagents are silicas, including gels and precipitates; insoluble sodiumpolymetaphosphate; β-phase calcium pyrophosphate; alumina and resinousabrasive materials which do not contribute soluble calcium ions whichform complexes with the fluoride ion. Calcium carbonate, calciumphosphate and regular calcium pyrophosphate are excluded.

U.S. Pat. No. 4,254,101 describes toothpastes containing a humectant,silica abrasive, a carboxyvinyl polymer, water and fluoride compounds asoptional ingredients. The only carboxyvinyl polymers disclosed arecolloidally water-soluble polymers of acrylic acid crosslinked withpolyallyl sucrose or polyallyl entaerythritol. A variety of fluoridecompounds including sodium fluoride are described as suitable optionalingredients. Stannous fluoride is included in the list of suitablefluoride compounds although no suggestion is made about its instabilityor that it could be used as an effective source of stannous ion. It ispresented as equivalent to sodium fluoride as a source of fluoride ion.Phosphorus-containing anticalculus agents are also listed as optionalingredients.

U.S. Pat. No. 4,515,772 describes oral compositions includingdentrifices and aqueous compositions including toothpastes andmouthwashes containing certain pyrophosphate salts as anticalculusformulations. In the prior art description, it lists a number ofchelating agents proposed as anticalculus agents, including EDTA(ethylenediaminetetraacetic acid), nitrilotriacetic acid,polyphosphonates and fluoride, and carbonyl diphosphonates. The patentlists as suitable abrasives silica, calcium pyrophosphate, β-phasepyrophosphate, alumina and other materials. A comprehensive list offluoride ion sources are listed. Stannous fluoride is included in thelist although no suggestion is made about its instability or that itcould be used as an effective source of stannous ion. A variety offlavoring agents are disclosed. Included in a list of binders are gumsand carboxyvinyl polymers.

U.S. Pat. No. 3,956,479 describes use of quaternary anticalculuscompounds in dental creams, tables or powders containing polishingagents such as water-insoluble phosphates, binders, detergents, gellingagents, flavoring agents, and fluoride-containing compounds such assodium fluoride, stannous fluoride and sodium monofluorophosphate.

U.S. Pat. No. 4,627,977 describes oral compositions containing acalculus-inhibiting amount of a linear molecularly dehydratedpolyphosphate salt, and to inhibit hydrolysis of the polyphosphate saltin the saliva, a combination of a fluoride ion-providing source and asynthetic linear polymeric polycarboxylate. Included in the list ofsuitable polycarboxylates are copolymers of maleic anhydride or acid andethylenically unsaturated monomers including alkyl vinyl ethersavailable as Grantrez AN 139, AN 119, and S97. An extensive list offluoride ion sources are provided, including sodium and stannousfluorides and sodium monofluorophosphate. An extensive list of polishingagents are provided including water-insoluble phosphates, silicates andsilicas, bentonites and inorganic polishing agents. Silicas are listedas preferred.

A. Gaffar et al in Compend.Contin.Educ.Dent, Suppl. No. 8, 242-250(1987) and Thomas G. Schiff in Compend.Contin.Educ.Dent, Suppl. No 8,275-277 (1987) describe evaluations of pyrophosphate compositionscontaining sodium fluoride and a copolymer of methoxyethylene and maleicanhydride known as GANTREZ®as anticalculus dentifrices. The oral cavitycontains enzymes which attack and degrade the pyrophosphates, rapidlyremoving their anticalculus activity. The enzymes require solublemagnesium ion for activity. The copolymer is provided to complexmagnesium ions, making them unavailable to the enzymes and thusinhibiting their activity. Its purpose is to protect the pyrophosphatesand maintain their anticalculus activity.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention provides an aqueous toothpaste compositioncontaining stannous fluoride in combination with a stannous ionchelating copolymer of an alkyl vinyl ether and maleic anhydride or acidin an amount to effectively stabilize the stannous ion. The compositionpreferably contains from 10 to 30 weight percent water, from 0.05 to 5and preferably from 0.5 to 5 weight percent of the chelating copolymerand from 0.05 to 5 and preferably from 0.3 to 0.5 weight percentstannous fluoride and an insoluble abrasive agent. The weight ratio ofstannous fluoride to copolymer is preferably between 0.01 to 1.0. Thecomposition is substantially free from silica, soluble phosphates suchas soluble pyrophosphates (i.e., tetrasodium pyrophosphate,tetrapotassium phosphates, etc.), and aldehyde group containingcompounds.

The method of this invention for formulating an aqueous toothpasteincluding stannous fluoride comprises the step of dispersing thestannous fluoride in an aqueous solution of an alkyl vinyl ether andmaleic anhydride or acid copolymer, the amount of said copolymer beingsufficient to stabilize the stannous fluoride concentration duringformulation.

It is an object of this invention to provide a stannous fluoridetoothpaste formulation which is "effectively stabilized", that is, thestannous fluoride concentration in the product after three months atforty-five degrees centigrade remains at an acceptable therapeuticlevel. Product stability after three months at forty-five degreescentigrade is comparable to product stability after three years at roomtemperature.

DETAILED DESCRIPTION OF THE INVENTION

The essential ingredients of the compositions of this invention arestannous fluoride and the stannous ion chelating copolymer in aneffectively stabilizing amount.

The term "effectively stabilized" and "effectively stabilizing amount"is defined to mean that the stannous ion concentration, expressed asstannous fluoride, after three months storage under the conditionsdescribed in Example 2 is equivalent to about 70 percent or more of theoriginal concentration of stannous ion at the time of formulation.

The composition contains from 10 to 30 weight percent water, from 0.05to 5 and preferably from 0.5 to 5 weight percent of the chelatingcopolymer and from 0.05 to 5 and preferably from 0.3 to 0.5 weightpercent stannous fluoride and an insoluble abrasive agent.

The stannous ion stabilizing copolymer is a copolymer of maleicanhydride or acid and a polymerizable ethylenically unsaturated monomer,preferably a lower alkyl vinyl ether such as methoxyethylene, having amolecular weight of from about 30,000 to 1,000,000. The mole ratios ofthe maleic anhydride or acid to the ethylenically unsaturated monomer ispreferably from 1:4 to 4:1. Suitable polymers are available from GAFunder the tradename GANTREZ™ and are disclosed in U.S. Pat. No.4,627,977, the entire contents of which are hereby incorporated byreference. These copolymers have the unique ability to form chelateswith the stannous ion which are sufficiently strong to provide oxidationprotection to the stannous ion while being sufficiently weak to notremove calcium from the tooth structure. EDTA and other strong chelatingagents are undesirable because they deplete calcium from the toothenamel.

The term "lower alkyl" is defined to be a straight or branch-chainedhydrocarbon group having from 1 to 6 carbons and preferably from 1 to 4carbons and including methyl, ethyl, propyl, isopropyl, butyl, isobutyl,t-butyl, pentyl and hexyl groups.

The toothpaste formulation contains one or more abrasive agents (otherthan silica or silicates) and can contain other conventional ingredientsto impart the desired consistency, texture, resistance to hardening,flavor and the like.

Suitable abrasive agents include insoluble phosphates such as calciumpyrophosphate, β-phase pyrophosphate, and alkali metal metaphosphates Wehave discovered that the stabilizing function of the stannous ionchelating polymer is not effective in the presence of silica andsilicates.

Any conventional humectant can be used. Suitable humectants includesorbitol, glycerin, propylene glycol, and other polyhydric alcohols; andthe natural or synthetic gums conventionally used as hardening controlagents and binders.

The compositions can also contain conventional flavoring agents which donot have an aldehyde group. We have discovered that aldehyde-containingflavoring agents such as cinnaldehyde, while not reacting directly withthe stannous ion, interacts with the stannous ion chelating polymer,allowing stannous oxidation to occur.

The compositions should have a pH within the range of from 2 to 11 andpreferably from 3 to 7.

The compositions can also contain other conventional components such asanti-foaming agents, anticariogenic agents, soluble fluoride compounds,surfactants, gelling agents, coloring or whitening agents, antibacterialagents, preservatives, chlorophyll compounds, and additional ammoniatedmaterials which do not interfere with the stability of the stannous ionby reacting therewith or with the stannous ion chelating polymer.Suitable materials are described in U.S. Pat. Nos. 4,254,101 and4,627,977, the entire contents of which are incorporated by reference intheir entireties.

The formulations of this invention are manufactured using procedureswhich protect the stannous ion from oxidation. The stannous fluoride isinitially dissolved in an aqueous solution containing the stannouschelating copolymer. This solution can be then mixed with othercomponents by conventional procedures to form the toothpasteformulations of this invention. In a preferred procedure, a premixsolution is prepared by dispersing from 0.5 to 26 weight percentstannous fluoride in water containing from 5.0 to 26 weight percentstannous ion stabilizing copolymer, the weight ratios of the stannousfluoride to copolymer being from 0.01 to 1.0 in the premix solution. Thepremix is then mixed with the other components of the toothpaste toprovide the desired level of stannous fluoride. Additional protectionfrom oxidation can be provided during manufacture by carrying out theprocedures in an inert atmosphere.

For additional protection against the oxidation, the product should bestored in oxygen impermeable containers. Preferred containers arelaminated tube such as GLAMINATE™ tubes (American Can Company). Suitabletubes are laminates of a flexible plastic such as a polyolefin (i.e.,low density polyethylene, etc.) and an oxygen barrier layer such as anoxygen impermeable metal foil (i.e., aluminum foil). The laminatesinclude other conventional layers such as adhesives (i.e., ethylenecopolymers), paper or other non-woven fibrous materials, and the like.The layer exposed to the stannous fluoride toothpaste should beessentially free from toxic substances and materials which would bereleased into the toothpaste during manufacture, storage or use andimpair the stability of the toothpaste composition.

Preferred formulations have the composition shown in Table A.

                  TABLE A                                                         ______________________________________                                        Ingredients          Weight Percent                                           ______________________________________                                        Purified Water USP   10-30                                                    Carbopol.sup.a       0.05-1.0                                                 Sodium CMC.sup.b     0.1-2.0                                                  Glycerin 96% USP     5.0-30                                                   Xanthan Gum (KELTROL)                                                                              0.1-1.5                                                  Methylparaben NF     0.1-5.0                                                  Propylparaben NF     0.01-.10                                                 Polyethylene Glycol 400.sup.c                                                                      0.1-4.0                                                  Stannous Chelating Copolymer                                                                       0.5-5.0                                                  Stannous Fluoride    0.3-0.5                                                  Sorbitol 70% USP     10-35                                                    Sodium Saccharin USP Crystal                                                                       0.05-0.5                                                 Calcium Pyrophosphate                                                                              30-45                                                    Zinc Citrate         0.1-2.0                                                  Insoluble Sodium Metaphosphate                                                                     30-45                                                    Sodium Lauryl Sulfate Needles                                                                      0.05-3.0                                                 Sodium Hydroxide NF (10% sol'n)                                                                    0.5-5.0                                                  Flavor               0.5-1.5                                                  ______________________________________                                         Carbopol.sup.a is a carboxyvinyl polymer available from B. F. Goodrich        Company.                                                                      Sodium CMC.sup.b is carboxymethyl cellulose.                                  Polyethylene Glycol 400.sup.c is polyethylene glycol having a molecular       weight of about 400, available from Union Carbide.                       

This invention is further described by the following specific butnon-limiting examples. Percentages are weight percents unless otherwiseindicated.

EXAMPLE 1

An aqueous solution containing methyl vinyl ether-maleic anhydridecopolymer and percent stannous fluoride is prepared to form a stabilizedstannous fluoride solution. This solution is mixed with the otheringredients listed in Table B below. The amounts of the ingredients areselected to yield a final toothpaste composition having the followingcomposition:

                  TABLE B                                                         ______________________________________                                        Ingredients            Weight Percent                                         ______________________________________                                        Purified Water         29.00                                                  Polyethylene Glycol 400                                                                              0.40                                                   Glycerin 96% USP       5.00                                                   Methylparaben NF       0.15                                                   Propylparaben NF       0.05                                                   Copolymer.sup.a        1.00                                                   Stannous Fluoride      0.455                                                  Sodium Hydroxide (10% solution)                                                                      4.50                                                   Sorbitol 70% USP       16.275                                                 Sodium Saccharin USP Crystal                                                                         0.20                                                   Calcium Pyrophosphate  40.00                                                  Zinc Citrate           0.40                                                   Glycerine 96% USP      5.00                                                   Natural Mint Flavor A-1159 FELTON                                                                    1.00                                                   Sodium Lauryl Sulfate Needles                                                                        1.00                                                   ______________________________________                                         Copolymer.sup.a is the free acid of a methyl vinyl ether and maleic           anhydride copolymer having a molecular weight of about 70,000,                benzenefree, and sold under the tradename GANTREZ S97BF by GAF                Corporation.                                                             

EXAMPLE 2

Dentrifrice compositions prepared as described in Example 1 and havingthe formulations shown in Table C were analyzed to determine the initialconcentrations of stannous and fluoride ions achieved at formulation adnto determine the initial ion concentrations after 1, 2, and 3 monthsstorage at 45° C. in sealed oxygen impermeable tubes.

                  TABLE C                                                         ______________________________________                                                     Weight Percent                                                   Ingredients    A       B        C     D                                       ______________________________________                                        Purified Water 29.00   29.00    29.00 29.00                                   Carbopol 950   0.20    0.20     0.20  0.20                                    Glycerin 96% USP                                                                             10.00   10.00    10.00 10.00                                   Xanthan Gum    0.15    0.40     0.40  0.40                                    Methylparaben NF                                                                             0.15    0.15     0.15  0.15                                    Propylparaben NF                                                                             0.02    0.02     0.02  0.02                                    Copolymer.sup.a                                                                              1.00    1.00     1.00  1.00                                    Stannous Fluoride                                                                            0.455   0.455    0.455 0.455                                   Sodium Hydroxide                                                                             4.50    4.50     4.50  4.50                                    (10% solution)                                                                Sorbitol 70% USP                                                                             21.225  10.275   16.675                                                                              16.675                                  Sodium Saccharin                                                                             0.20    0.20     0.20  0.20                                    USP Crystal                                                                   Calcium Pyrophosphate                                                                        30.00   35.00    35.00 35.00                                   Zinc Citrate   0.80    0.80     0.40  0.40                                    Natural Mint Flavor                                                                          1.00    1.00                                                   A-1159 FELTON                                                                 Doublemint Flavor               1.00  1.00                                    #30672 Noville                                                                Sodium Lauryl Sulfate                                                                        1.00    1.00     1.00  1.00                                    Needles                                                                       ______________________________________                                         Copolymer.sup.a is the free acid of a methyl vinyl ether and maleic           anhydride copolymer having a molecular weight of about 70,000,                benzenefree, and sold under the tradename GANTREZ S97BF by GAF                Corporation.                                                             

The following results were obtained, concentrations being reported asweight percents.

    ______________________________________                                        Time       Sn as SnF.sub.2                                                                            F as SnF.sub.2                                                                            pH                                        ______________________________________                                        Formulation A                                                                 Initial    0.453        0.420       5.2                                       one month  0.435        0.439       5.2                                       two months 0.458        0.428       5.1                                       three months                                                                             0.412        0.414       5.2                                       Formulation B                                                                 Initial    0.479        0.430       5.1                                       one month  0.444        0.442       5.1                                       two months 0.402        0.398       5.2                                       three months                                                                             0.433        0.353       5.2                                       Formulation C                                                                 Initial    0.454        0.452       5.3                                       one month  0.439        0.472       5.2                                       two months 0.468        0.472       5.2                                       three months                                                                             0.477        0.469       5.3                                       Formulation D                                                                 Initial    0.447        0.429       5.7                                       one month  0.443        0.440       5.6                                       two months 0.455        0.420       5.8                                       three months                                                                             0.458        0.441       5.8                                       ______________________________________                                    

EXAMPLE 3

Dentifrice compositions prepared as described in Example 1 and havingthe formulations shown in Table D were analyzed to determine the initialconcentrations of stannous and fluoride ions achieved at formulation andto determine the initial ion concentrations after 1, 2, and 3 monthsstorage at 45° C. in sealed oxygen impermeable tubes.

                                      TABLE D                                     __________________________________________________________________________                     Weight Percent                                               Ingredients      E   F   G   H   I                                            __________________________________________________________________________    Purified Water   29.50                                                                             22.50                                                                             29.50                                                                             24.50                                                                             17.00                                        Carbopol 950     0.30    0.20                                                                              0.20                                             Glycerin 96% USP 10.00                                                                             10.00                                                                             10.00                                                                             10.00                                                                             10.00                                        Xanthan Gum      0.50                                                                              0.60                                                                              0.80                                                                              0.80                                                                              0.70                                         Carboxymethyl Cellulose                                                                            0.40        0.30                                         Methylparaben NF 0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                         Propylparaben NF 0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                         Copolymer.sup.a  1.00                                                                              1.00                                                                              1.00                                                                              1.00                                                                              1.00                                         Stannous Fluoride                                                                              0.455                                                                             0.455                                                                             0.455                                                                             0.455                                                                             0.43                                         Sodium Hydroxide (10% solution)                                                                4.50                                                                              3.00                                                                              4.50                                                                              4.00                                                                              3.00                                         Sorbitol 70% USP 14.975                                                                            24.275                                                                            14.975                                                                            15.875                                                                            24.40                                        Sodium Saccharin USP Crystal                                                                   0.20                                                                              0.20                                                                              0.20                                                                              0.20                                                                              0.20                                         Calcium Pyrophosphate                                                                          35.00                                                                             35.00                                                                             35.00                                                                             40.00                                                                             40.00                                        Zinc Citrate     0.40                                                                              0.40                                                                              0.40                                                                              0.80                                                                              0.80                                         Tween-80.sup.b   1.0                                                          Pluronic F127.sup.c      0.80                                                 Mint Flavor A-1159 FELTON                                                                      0.8 0.8 0.80                                                                              1.00                                                                              1.00                                         Sodium Lauryl Sulfate Needles                                                                  1.2 1.2 1.2 1.00                                                                              1.00                                         __________________________________________________________________________     Copolymer.sup.a is the free acid of a methyl vinyl ether and maleic           anhydride copolymer having a molecular weight of about 70,000,                benzenefree, and sold under the tradename GANTREZ S97BF by GAF                Corporation.                                                                  Tween80.sup.b is polyoxyethylenesorbitan monooleate.                           Pluronic F127.sup.c is a condensate of ethylene oxide with propylene         oxide condensates of propylene glycol.                                   

The following results were obtained, concentrations being reported asweight percents.

    ______________________________________                                        Time       Sn as SnF.sub.2                                                                            F as SnF.sub.2                                                                            pH                                        ______________________________________                                        Formulation E                                                                 Initial    0.462        0.453       5.6                                       one month  0.432        0.449       5.6                                       two months 0.453        0.444       5.4                                       three months                                                                             0.431        0.453       5.4                                       Formulation F                                                                 Initial    0.471        0.435       5.2                                       one month  0.390        0.441       5.2                                       two months 0.415        0.436       5.0                                       three months                                                                             0.418        0.429       5.0                                       Formulation G                                                                 Initial    0.456        0.449       5.8                                       one month  0.445        0.439       5.7                                       two months 0.451        0.449       5.5                                       three months                                                                             0.454        0.430       5.4                                       Formulation H                                                                 Initial    0.462        0.460       5.4                                       one month  0.464        0.438       5.1                                       two months 0.459        0.440       5.1                                       three months                                                                             0.450        0.445       4.9                                       Formulation I                                                                 Initial    0.469        0.438       5.2                                       one month  0.396        0.407       4.9                                       two months 0.414        0.410       4.9                                       three months                                                                             0.398        0.409       4.6                                       ______________________________________                                    

EXAMPLE 4

The following Example illustrates the stabilizing effect of the alkylvinyl ether/maleic anhydride copolymers on stannous fluoride in anaqueous environment such as encountered during formulation of stannousfluoride containing oral care products. Two sample solutions of 0.455percent by weight stannous fluoride in deionized water were prepared. Inthe first sample solution, the stannous fluoride was added to thedeionized water. In a second sample solution, the stannous fluoride wasadded to a solution of 1 percent by weight of methyl vinyl ether-maleicanhydride copolymer in deionized water. In the first sample solution aprecipitate of stannous oxides appeared almost immediately and theintensity of the precipitate increased with time. Analyses of thesupernatant and sediment portions of centrifuged samples revealed thatthe concentration of stannous fluoride in the supernatant and sedimentportions were 0.371 percent by weight and 0.099 percent by weightrespectively. The second sample solution was clear and there was noevidence of precipitation. These results were confirmed by comparativesample analyses using a turbidity meter. Analyses of the supernatant andsediment portions of centrifuged samples revealed that the concentrationof stannous fluoride in the supernatant was 0.456 percent by weight andno stannous fluoride was detected in the sediment portion.

What is claimed is:
 1. In a method for formulating an aqueous toothpasteconsisting essentially of water-unstable stannous fluoride, theimprovement comprising the step of dispersing the stannous fluoride inan aqueous solution of an alkyl vinyl ether and maleic anhydride ormaleic acid copolymer as the essential stannous ion chelate formingstabilizer, the amount of said copolymer being sufficient to stabilizethe stannous fluoride concentration during formulation against stannousoxide precipitation by providing oxidation protection to the stannousion.
 2. A method of claim 1 wherein the alkyl vinyl ether is methylvinyl ether.
 3. A method of claim 1 where the amount of stannousfluoride included in the solution can provide between about 0.05 toabout 5.0 percent by weight stannous fluoride in the formulatedtoothpaste composition.